Old CDs with bad DC offset

Earlier I opened up a few old cds with that warning about the audio being imperfect because of an analog source. I wondered if audacity could make them sound nicer, and when I imported one it was easy to see that it had terrible dc offset. When removed the songs sound much crisper – of course this could be the placebo effect, so could someone please explain to me what removing dc offset does perceptually? Also it is stereo and the tracks are offset in different directions.
On another note there are frequent volume spikes that coincide with a snare hit but the snare is quite quiet. Is there a way of removing the spikes without affecting the rest of the audio?

If the spikes are well beyond the rest of the envelope the (free) “popmute” plug-in is worth a try.

You can’t hear DC offset. One of the early complaints I had about Audacity was it would cheerfully edit and manage battery voltage as if it were sound…and it’s not.

That’s not to say DC voltage in the show can’t cause other problems. If it’s bad enough, it can cause speaker amplifiers and speakers themselves to misbehave, if the error gets that far. That will be very clearly audible.

It can also cause peak distortion because one side of the audio waveform will distort and clip before the other. That, too, is clearly audible, but these are secondary symptoms.

One thing that DC Offset does is make editing, cutting, and sound management all but impossible. Every edit may pop and click. It’s highly desirable to get rid of it as soon as possible and you need to do it before you start editing.


Audio from CDs should not have DC off-set, regardless of the original recording.
How have you extracted the audio from the CDs?

I ripped it to alac with itunes and then opened the tracks in audacity. Most of my other itunes cds do not have this problem. And thanks for the tip with the popping.

That should produce an accurate copy of the CD data, which suggests that whoever made the CD messed up.

DC offset can usually be corrected by using the Normalize effect with
Remove DC offset = selected
Normalize maximum amplitude = Not selected.

If imported files show signs of DC offset you should correct it as the first step.